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Chapter 13 - meiosis and sexual life cycles.

Chapter 13 Meiosis and Sexual Life Cycles Lecture Outline

Overview: Hereditary Similarity and Variation

• Living organisms are distinguished by their ability to reproduce their own kind.
• Offspring resemble their parents more than they do less closely related individuals of the same species.
• The transmission of traits from one generation to the next is called heredity or inheritance.
• However, offspring differ somewhat from parents and siblings, demonstrating variation.
• Farmers have bred plants and animals for desired traits for thousands of years, but the mechanisms of heredity and variation eluded biologists until the development of genetics in the 20th century.
• Genetics is the scientific study of heredity and variation.

Concept 13.1 Offspring acquire genes from parents by inheriting chromosomes

• Your genome is comprised of the tens of thousands of genes that you inherited from your mother and your father.
• Genes program specific traits that emerge as we develop from fertilized eggs into adults.
• This is analogous to the symbolic information of language in which words and sentences are translated into mental images.
• Cells translate genetic “sentences” into freckles and other features with no resemblance to genes.
• Most genes program cells to synthesize specific enzymes and other proteins whose cumulative action produces an organism’s inherited traits.
• This produces copies of genes that can be passed from parents to offspring.
• In plants and animals, sperm and ova (unfertilized eggs) transmit genes from one generation to the next.
• After fertilization (fusion of a sperm cell and an ovum), genes from both parents are present in the nucleus of the fertilized egg, or zygote.
• Tiny amounts of DNA are also found in mitochondria and chloroplasts.
• Humans have 46 chromosomes in almost all of their cells.
• Each chromosome consists of a single DNA molecule associated with various proteins.

Like begets like, more or less: a comparison of asexual and sexual reproduction.

• Only organisms that reproduce asexually can produce offspring that are exact copies of themselves.
• Single-celled eukaryotes can reproduce asexually by mitotic cell division to produce two genetically identical daughter cells.
• Some multicellular eukaryotes, like Hydra, can reproduce by budding, producing a mass of cells by mitosis.
• Members of a clone may be genetically different as a result of mutation.
• In sexual reproduction, two parents produce offspring that have unique combinations of genes inherited from the two parents.
• Unlike a clone, offspring produced by sexual reproduction vary genetically from their siblings and their parents.

Concept 13.2 Fertilization and meiosis alternate in sexual life cycles

• A life cycle is the generation-to-generation sequence of stages in the reproductive history of an organism.

Human cells contain sets of chromosomes.

• Each chromosome can be distinguished by size, position of the centromere, and pattern of staining with certain dyes.
• The two chromosomes comprising a pair have the same length, centromere position, and staining pattern.
• These homologous chromosome pairs carry genes that control the same inherited characters.
• Two distinct sex chromosomes, the X and the Y, are an exception to the general pattern of homologous chromosomes in human somatic cells.
• The other 22 pairs are called autosomes.
• Human females have a homologous pair of X chromosomes (XX).
• Human males have an X and a Y chromosome (XY).
• Most of the genes carried on the X chromosome do not have counterparts on the tiny Y.
• The Y chromosome also has genes not present on the X.
• The occurrence of homologous pairs of chromosomes is a consequence of sexual reproduction.
• The 46 chromosomes in each somatic cell are two sets of 23, a maternal set (from your mother) and a paternal set (from your father).
• The number of chromosomes in a single set is represented by n.
• Any cell with two sets of chromosomes is called a diploid cell and has a diploid number of chromosomes, abbreviated as 2n.
• Sperm cells or ova (gametes) have only one set of chromosomes—22 autosomes and an X (in an ovum) and 22 autosomes and an X or a Y (in a sperm cell).
• A gamete with a single chromosome set is haploid, abbreviated as n.
• For humans, the haploid number of chromosomes is 23 (n = 23), and the diploid number is 46 (2n = 46).

Let’s discuss the role of meiosis in the human life cycle.

• The human life cycle begins when a haploid sperm cell fuses with a haploid ovum.
• These cells fuse (syngamy), resulting in fertilization.
• The fertilized egg (zygote) is diploid because it contains two haploid sets of chromosomes bearing genes from the maternal and paternal family lines.
• Each somatic cell contains a full diploid set of chromosomes.
• If gametes were produced by mitosis, the fusion of gametes would produce offspring with four sets of chromosomes after one generation, eight after a second, and so on.
• Human sperm or ova have a haploid set of 23 different chromosomes, one from each homologous pair.

Organisms display a variety of sexual life cycles.

• Fertilization and meiosis alternate in all sexual life cycles.
• However, the timing of meiosis and fertilization does vary among species.
• These variations can be grouped into three main types of life cycles.
• Gametes do not divide but fuse to form a diploid zygote that divides by mitosis to produce a multicellular organism.
• This life cycle includes two multicellular stages, one haploid and one diploid.
• The multicellular diploid stage is called the sporophyte.
• Meiosis in the sporophyte produces haploid spores that develop by mitosis into the haploid gametophyte stage.
• Gametes produced via mitosis by the gametophyte fuse to form the zygote, which grows into the sporophyte by mitosis.
• Gametes fuse to form a zygote, which is the only diploid phase.
• The zygote undergoes meiosis to produce haploid cells.
• These haploid cells grow by mitosis to form the haploid multicellular adult organism.
• The haploid adult produces gametes by mitosis.
• Note that either haploid or diploid cells can divide by mitosis, depending on the type of life cycle. However, only diploid cells can undergo meiosis.
• Although the three types of sexual life cycles differ in the timing of meiosis and fertilization, they share a fundamental feature: each cycle of chromosome halving and doubling contributes to genetic variation among offspring.

Concept 13.3 Meiosis reduces the number of chromosome sets from diploid to haploid

• Both are preceded by the replication of chromosomes.
• The first division, meiosis I, separates homologous chromosomes.
• The second, meiosis II, separates sister chromatids.
• The four daughter cells have only half as many chromosomes as the parent cell.
• These are genetically identical and joined at the centromere.
• The single centrosome is replicated, forming two centrosomes.
• Prophase I typically occupies more than 90% of the time required for meiosis.
• During prophase I, the chromosomes begin to condense.
• In crossing over, DNA molecules in nonsister chromatids break at corresponding places and then rejoin the other chromatid.
• In synapsis, a protein structure called the synaptonemal complex forms between homologues, holding them tightly together along their length.
• As the synaptonemal complex disassembles in late prophase, each chromosome pair becomes visible as a tetrad, or group of four chromatids.
• Each tetrad has one or more chiasmata, sites where the chromatids of homologous chromosomes have crossed and segments of the chromatids have been traded.
• Spindle microtubules form from the centrosomes, which have moved to the poles.
• The breakdown of the nuclear envelope and nucleoli take place.
• Kinetochores of each homologue attach to microtubules from one of the poles.

Metaphase I

• Microtubules from one pole are attached to the kinetochore of one chromosome of each tetrad, while those from the other pole are attached to the other.
• Sister chromatids remain attached at the centromere and move as a single unit toward the pole.

Telophase I and cytokinesis

• Each chromosome consists of two sister chromatids.
• In animal cells, a cleavage furrow forms. In plant cells, a cell plate forms.
• Spindle fibers from one pole attach to the kinetochore of one sister chromatid, and those of the other pole attach to kinetochore of the other sister chromatid.
• Because of crossing over in meiosis I, the two sister chromatids of each chromosome are no longer genetically identical.
• The kinetochores of sister chromatids attach to microtubules extending from opposite poles.
• At anaphase II, the centomeres of sister chromatids separate and two newly individual chromosomes travel toward opposite poles.
• Nuclei form around the chromosomes, which begin expanding, and cytokinesis separates the cytoplasm.

There are key differences between mitosis and meiosis.

• The chromosome number is reduced from diploid to haploid in meiosis but is conserved in mitosis.
• Mitosis produces daughter cells that are genetically identical to the parent and to each other.
• Meiosis produces cells that are genetically distinct from the parent cell and from each other.
• During prophase I of meiosis, replicated homologous chromosomes line up and become physically connected along their lengths by a zipperlike protein complex, the synaptonemal complex, in a process called synapsis. Genetic rearrangement between nonsister chromatids called crossing over also occurs. Once the synaptonemal complex is disassembled, the joined homologous chromosomes are visible as a tetrad. X-shaped regions called chiasmata are visible as the physical manifestation of crossing over. Synapsis and crossing over do not occur in mitosis.
• At metaphase I of meiosis, homologous pairs of chromosomes align along the metaphase plate. In mitosis, individual replicated chromosomes line up along the metaphase plate.
• At anaphase I of meiosis, it is homologous chromosomes, not sister chromatids, that separate and are carried to opposite poles of the cell. Sister chromatids of each replicated chromosome remain attached. In mitosis, sister chromatids separate to become individual chromosomes.
• Meiosis I is called the reductional division because it halves the number of chromosome sets per cell—a reduction from the diploid to the haploid state.
• The sister chromatids separate during the second meiosis division, meiosis II.

Concept 13.4 Genetic variation produced in sexual life cycles contributes to evolution

• What is the origin of genetic variation?
• Mutations are the original source of genetic diversity.

Sexual life cycles produce genetic variation among offspring.

• The behavior of chromosomes during meiosis and fertilization is responsible for most of the variation that arises in each generation.
• Independent assortment of chromosomes.
• Crossing over.
• Random fertilization.
• There is a fifty-fifty chance that a particular daughter cell of meiosis I will get the maternal chromosome of a certain homologous pair and a fifty-fifty chance that it will receive the paternal chromosome.
• Each homologous pair of chromosomes segregates independently of the other homologous pairs during metaphase I.
• Therefore, the first meiotic division results in independent assortment of maternal and paternal chromosomes into daughter cells.
• If n = 3, there are 23 = 8 possible combinations.
• For humans with n = 23, there are 223, or more than 8 million possible combinations of chromosomes.
• Crossing over produces recombinant chromosomes, which combine genes inherited from each parent.
• Crossing over begins very early in prophase I as homologous chromosomes pair up gene by gene.
• For humans, this occurs an average of one to three times per chromosome pair.
• Recent research suggests that, in some organisms, crossing over may be essential for synapsis and the proper assortment of chromosomes in meiosis I.
• Crossing over, by combining DNA inherited from two parents into a single chromosome, is an important source of genetic variation.
• At metaphase II, nonidentical sister chromatids sort independently from one another, increasing by even more the number of genetic types of daughter cells that are formed by meiosis.
• The random nature of fertilization adds to the genetic variation arising from meiosis.
• The ovum is one of more than 8 million possible chromosome combinations.
• The successful sperm is one of more than 8 million possibilities.
• The resulting zygote could contain any one of more than 70 trillion possible combinations of chromosomes.
• Crossing over adds even more variation to this.
• Each zygote has a unique genetic identity.
• Independent assortment of homologous chromosomes during meiosis I and of nonidentical sister chromatids during meiosis II.
• Crossing over between homologous chromosomes during prophase I.
• Random fertilization of an ovum by a sperm.

Evolutionary adaptation depends on a population’s genetic variation.

• A population evolves through the differential reproductive success of its variant members.
• Those individuals best suited to the local environment leave the most offspring, transmitting their genes in the process.
• This natural selection results in adaptation, the accumulation of favorable genetic variations.
• The formerly favored genes will decrease.
• Sex and mutation continually generate new genetic variability.
• Although Darwin realized that heritable variation makes evolution possible, he did not have a theory of inheritance.
• However, this work was largely unknown until 1900, after Darwin and Mendel had both been dead for more than 15 years.

Lecture Outline for Campbell/Reece Biology, 7th Edition, © Pearson Education, Inc. 13-1

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13: Chromosomes, Mapping, and the Meiosis-Inheritance Connection

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• 13.1: Sex Linkage and the Chromosomal Theory of Inheritance
• 13.2.1: Sex Chromosomes
• 13.2.2: Inferring the Mode of Inheritance
• 13.3.1: Organellar Inheritance
• 13.4.1: Classification and Detection of Molecular Markers
• 13.4.2: Genetic Linkage and Distances
• 13.5.1: Imprinted Genes
• 13.5.2: Prenatal Screening

Campbell Mastering Biology Chapter 27 Questions Flashcards

Why is salt a good preservative to use for foods such as pork and fish?

Prokaryotic cells living in the food will shrink from their cell walls, impacting their ability to reproduce.

Gram-negative bacteria have ________ peptidoglycan than gram-positive cells, and their cell walls are __________ complex structurally.

Less...More

A gram-negative cell wall consists of _____________.

a thin layer of peptidoglycan surrounded by an outer membrane containing lipopolysaccharides.

Bacteria that ________ tend to have abundant internal membranes.

are photosynthetic

Bacterial cells, but not eukaryotic cells, possess ______________.

a nucleoid with a circular chromosome

Bacterial flagella have a very complex structure composed of 42 distinct proteins. What s the most likely explanation for the evolution of these complex structures?

The bacteria that cause tetanus can be killed only by prolonged heating at temperatures considerably above boiling. This suggests that these bacteria _______________.

produce endospores

Plasmids ___________.

-Replicate independently of the main chromosome -Allow bacteria to survive adverse conditions -Often contain antibiotic resistance genes -Are transferred from on bacterium to another by conjugation. (All the listed responses are correct)

How is it possible that as many as 9 million mutations can arise each day in the E. coli inhabiting one human?

The mathematics of large population size and rapid reproduction rate combine to produce many mutations without a particularly high mutation rate.

In the absence of meiosis and sexual reproduction, what general process allows genetic recombination among prokaryotes?

Horizontal gene transer

Which statement about transformation is true?

It can be facilitated by cell-surface proteins that recognize compatible DNA.

An F+ bacterial cell _____________.

acts as a donor during conjugation

Which of the following is true about R plasmids?

-They can be transferred from one bacterium to another via conjugation. -They can carry several resistance genes. (Both the second and third answers are correct)

Bacteria that use light for their energy source and CO2 for their carbon source are called ________________.

Photo-autotrophs

In an experiment, a microbiologist put equal numbers of each of the following organisms into a flask of sterile north, consisting mostly of sugar and a few amino acids. She the placed the flask in the dark. Which of the organisms would be most likely to survive?

Chemo-heterotrophic bacteria

The Desulfovibrio bacterium breaks down organic matter (which it must have) and uses sulfate (not oxygen) as an electron acceptor. As a result, it produces hydrogen sulfide (H2S), accounting for the "rotten egg" smell of swamp muck. Oxygen is a deadly poison to Desulfovibrio. We would call Desullfovibrio a(n) _________.

Obligately anaerobic chemoautotroph

Choose the list below that contains the substances required by typical nitrogen-fixing cyanobacteria.

carbon dioxide, nitrogen, water, light, and some minerals

What is the role of heterocysts in a cyanobacterial filament?

They carry out only nitrogen fixation.

Biofilms are an example of ____________.

metabolic cooperation among prokaryotic species

Portions of the genomes of certain prokaryotic species are very similar to portions of the genomes of distantly related prokaryotes. The process that most likely accounts for this genetic similarity is _______________.

horizontal gene transfer

Which subgroup of proteobacteria contains many species that are closely associated with eukaryotic hosts in mutualistic or parasitic relationships?

Which subgroup of proteobacteria contains many species that are predators of other bacteria?

Which group of bacteria is unusual in that they lack peptidoglycan in their cell walls?

Which of the following is a difference between bacteria and archaea?

They have different chemicals in their cell membranes and cell walls.

Which of the following statement about cyanobacteria is true?

-Some are single cells, whereas other live in filamentous colonies. -They are the only prokaryotes that perform plantlike, oxygenic photosynthesis. -It can be said that nitrogen-fixing cyanobacteria are the most self-sufficient of all organisms. -Some species may carry on nitrogen fixation. (All of the listed responses are correct)

Prokaryotes found inhabiting the Great Salt Lake would be ____________.

extreme halophiles

Which of the flowing groups of prokaryotes is classified as a member of the domain Archaea?

methanogens

Which clade of archaea includes most of the extreme thermophiles?

Crenarchaeota

Prokaryotes are completely indispensable to which chemical cycle?

A type of ecological relationship called _______ involves one organism living at the expense of another organism.

Which example below is a correct statement about Bacteroides thetaoitaomicron, a bacterium that lives in the human intestines?

The bacteria have a mutualistic relationship with the human body.

Ticks that live on deer and field mice are responsible for spreading the bacterium ________, which causes ___________.

Borrelia burgdorferi....Lyme disease

Which statement is true regarding cholera?

Its symptoms are caused by an exotoxin that stimulates intestinal cells to release chloride ions into the gut.

Scientists hypothesize that the O157:H7 strain of E. coli is to different from the K-12 strain because of ___________.

horizontal gene transfer over many years, most likely through the action of bacteriophages

Which statement about prokaryotes is true?

Prokaryotes are widely used for bioremediation.